1. Field of the Invention
The present invention relates to power plants for generating electrical power and more particularly to combined gas and steam turbine plants capable of utilizing low BTU (British Thermal Unit) gas.
2. Description of the Prior Art
The abundance of coal in the United States and dwindling domestic oil reserves has created an increasing demand for the use of coal as a source of fuel for electrical power plants. While prior art power plants employing coal as the primary fuel have been in use for many years and are capable of operating with heat rates close to 9000 BTU/Kwh (kilowats hour), such plants present maintenance and particularly troublesome environmental problems.
The transformation of coal into a gaseous phase through a coal gasification process combined with a gaseous clean up process step provides a fuel which is cleaner and more readily usable than a solid coal. A combined coal gasification process and power generation plant referred to as the "Integrated Gasifier Combined Cycle ("IGCC") has recently been accepted as a standard approach for current studies of coal fueled power plants. The IGCC provides gasified coal at a high temperature to a gas turbine. Waste heat from the gas turbine is supplied to a low pressure steam turbine with the majority of the electrical energy being generated by the gas turbine. While the IGCC solves the environmental problems of conventional coal fired power plants, it has certain disadvantages.
To obtain heat rates of the order of 9000 BTU per kwh or below, combustion turbines capable of achieving a 2400.degree. F. (Fahrenheit) firing temperature are required. However, continuous operation of commercially available power plant turbines at such high temperatures have not been adequately demonstrated. In addition, most available gasifiers require oxygen feed, operating pressures above the available gas turbine combustor pressures and large gas treating facilities (e.g. of the type used in large petrochemical plants) for providing dry, clean fuel to the gas turbine. In addition, the waste heat steam turbines are operating at low efficiencies (due to the relatively low exhaust temperatures of the gas turbines) as compared to state of the art high temperature and high pressure reheat and extraction steam turbines.
There is need for an efficient power cycle capable of utilizing the low BTU gas derived from air blown coal gasifiers.